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Basics of Electricity

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Basics of Electricity Basics of Electricity Generator Theory PJM State & Member Training Dept. PJM©2014 8/6/2013 Objectives The student will be able to: • Describe the process of electromagnetic induction • Identify the major components of an AC generator • Apply the formula for rotational speed • Describe generator governor control • Discuss the characteristics that affect or limit generator performance • Describe MVAR and MW flow • Describe the synchronization process of a generator PJM©2014 8/6/2013 Basic Operating Principles • Electromagnetic Induction is the principle used by a generator to convert mechanical energy into electrical energy • For this to happen, three things are needed: • A magnetic field • A current‐carrying conductor • A relative motion between the two • This is the fundamental operating principle of generators, motors, transformers, inductors, and solenoids PJM©2014 8/6/2013 Basic Operating Principles • Generators use energy from the prime mover to turn the generator’s rotor (moving portion) inside the generator’s stator (static portion) • Steam in a fossil or nuclear unit • Water flow in a hydroelectric unit • Fuel flow in a combustion turbine unit • Wind flow in a wind generator • A DC current flows through the field windings of the rotor creating a magnetic field • As the rotor spins, it acts via electromagnetic induction to induce a voltage in the armature windings of the stator PJM©2014 8/6/2013 Basic Operating Principles • Output voltage of the generator is controlled by changing the strength of the magnetic field of the rotor. • This is accomplished by controlling the amount of direct current (DC) or excitation current that is supplied to the rotor’s field winding • The excitation current is supplied by the exciter • The stator’s output to the system is a three‐phase alternating current (AC) since the direction of the magnetic field changes in relation to the windings as the rotor turns 360 degrees • One rotation is equal to a complete cycle of power PJM©2014 8/6/2013 Basic Operating Principles Bismark Phasor Sim PJM©2014 8/6/2013 Basic Operating Principles PJM©2014 8/6/2013 A.C. Generator Components: Rotor • Rotating Magnetic Field • Series of Conductors PJM©2014 8/6/2013 A.C. Generator Components: Rotor • The basic function of the rotor is to produce a magnetic field of the size and shape necessary to induce the desired output voltage in the stator • The rotating field is required to produce a given number of lines of magnetic flux which is obtained by: Ampere‐turns • Ampere‐turns is the product of the number of turns in the rotor winding and the current that flows in the winding PJM©2014 8/6/2013 A.C. Generator Components: Rotor • Generator rotors are made of solid steel forgings with slots cut along the length for the copper windings • Insulated winding bars are wedged into the slots and connected at each end of the rotor and are arranged to act as one continuous wire to develop the magnetic field PJM©2014 8/6/2013 A.C. Generator Components: Rotor • Two types of rotors: • Salient‐pole • Used in low or moderate speed machines with separate field coils • Speeds from 100 to 1,200 rpm • Primarily for hydroelectric facilities • Cylindrical • Used in high‐speed machines that utilize a slotted integral forging • Speeds 1,200 rpm and above • Primarily used for steam and combustion turbine facilities PJM©2014 8/6/2013 A.C. Generator Components: Rotor • Rotor design constraints include: • Temperature: • Ampere‐turn requirements for the field increase with an increase in rating, which entails a combined increase in heating in the coil • Mechanical force: • Ampere‐turn requirements for the field increase with an increase in rating causing a higher centrifugal load • Electrical insulation: • In older units, slot insulation is a primary thermal barrier, and as current increases, becomes a greater obstacle PJM©2014 8/6/2013 A.C. Generator Components: Rotor Advantage: • Air gap between the stator and rotor can be adjusted so that the magnetic flux can be sinusoidal including the waveform Disadvantage: • Because of its weak structure it is not suitable for high‐ speed generation • It is also expensive to fabricate • Requires damper windings to prevent rotor oscillations during operations • Due to low speed, they are constructed with a higher number of poles to achieve system frequency PJM©2014 8/6/2013 A.C. Generator Components: Rotor Advantage: • Cheaper than a salient‐pole • Its symmetrical shape, is better for high‐ speed application • Losses in the windings are reduced • Noise produced is less Disadvantage: • Air gap is uniform • Generated voltage is polygonal giving way to the susceptibility of harmonics PJM©2014 8/6/2013 A.C. Generator Components: Rotor PJM©2014 8/6/2013 A.C. Generator Components: Stator PJM©2014 8/6/2013 A.C. Generator Components: Stator • The stator is the stationary part of the generator that is comprised of a series of stationary windings or “poles”(phases) • Basic function of the stator core is to provide a return path for the lines of magnetic flux, and support the coils of the stator winding • The rstato core is made of soft iron to provide the magnetic field path with a high permeability • The iron is laminated to reduce eddy currents (opposing field) and hysteresis (power losses) PJM©2014 8/6/2013 A.C. Generator Components: Stator • Two‐Pole Generators: • In a two‐pole generator, there are three armature winding coils installed in the stator • North and south poles of the rotor are 1800 apart • Four‐Pole Generators: • In a four‐pole generator, there are six armature winding coils installed in the stator • North and south poles of the rotor are 900 apart • A generator which is connected to the grid has a constant speed dictated by grid frequency Bismark AC Generator Sim PJM©2014 8/6/2013 A.C. Generator Components: Exciter PJM©2014 8/6/2013 A.C. Generator Components: Exciter • The function of the excitation system is to provide direct current for the generator rotor/field windings through slip rings to produce the magnetic field • Maintains generator voltage, controls MVAR flow, and assists in maintaining power system stability • During load changes or disturbances on the system, the excitert mus respond, sometimes rapidly, to maintain the proper voltage at the generator terminals PJM©2014 8/6/2013 Generator Rotational Speed • Frequency is dependent on: • Number of field poles • Speed of the generator • f = (N)(P)/120, where f = frequency (Hz) N = rotor speed (rpm) P = total number of poles 120 = Conversion from minutes to seconds and from “poles” to “pole pairs” (60 seconds/1 minute) x (2 poles/pole pair) PJM©2014 8/6/2013 Generator Rotational Speed Example: 2 Poles 60 Hz = (3600 RPM)(2 Poles)/120 PJM©2014 8/6/2013 Generator Rotational Speed Example: 4 Poles 60 Hz = (1800 RPM)(4 Poles)/120 PJM©2014 8/6/2013 Generator Governor Control • Governors control generator shaft speed • Adjust generation for small changes in load • Operate by adjusting the input to the prime mover • Steam flow for fossil • Water flow for hydro • Fuel flow for combustion turbine • Amount of governor control varies according to plant design • Equivalent to a car’s cruise control PJM©2014 8/6/2013 Generator Governor Control • The Watt centrifugal governor was the mechanical means for governor control • Used weights that moved radially as rotational speed increased that pneumatically operated a servo‐motor • Electrohydraulic governing has replaced the mechanical governor because of: • High response speed • Low deadband • Accuracy in speed and load control PJM©2014 8/6/2013 Generator Governor Control PJM©2014 8/6/2013 Generator Governor Control •Load • Rate of frequency decline from points A to C is slowed by “load rejection” • Generators • Generator governor action halts the decline in frequency and causes the “knee” of the excursion, and brings the frequency back to point B from point C It is important to note that frequency will not recover from point B to 60 Hz until the deficient control area replaces the amount of lost generation PJM©2014 8/6/2013 Generator Governor Control: Droop • Adding a droop characteristic to a governor forces generators to respond to frequency disturbances in proportion to their size • Droop settings enable many generators to operate in parallel while all are on governor control and not compete with one another for load changes PJM©2014 8/6/2013 Generator Governor Control: Droop PJM©2014 8/6/2013 Generator Governor Control: Droop PJM©2014 8/6/2013 Generator Governor Control: Deadband • Deadband • An additional feature displayed by governors • The amount of frequency change a governor must “see” before it starts to respond • Really a natural feature of the earliest governors caused by friction and gear lash (looseness or slop in the gear mechanism) • Serves a useful purpose by preventing governors from continuously “hunting” as frequency varies ever so slightly PJM©2014 8/6/2013 Generator Characteristics • Generator limitation factors • Power capability of the prime mover • Heating of generator components (I2R losses) • Necessity to maintain a strong enough magnetic field to transfer power from the rotor to the generator output PJM©2014 8/6/2013 Generator Characteristics • Heating of generator components • Heat generated within the armature and field windings is directly related to the magnitude of current flowing through them • Generator losses: • Based on resistance of the field winding of the rotor • Based on resistance and inductive reactance of the armature windings of the stator causing a voltage drop that subtracts from the output voltage • Heat dissipated by the generator is limited by the cooling system design PJM©2014 8/6/2013 Generator Characteristics • Magnetic field strength • Controlled by excitation voltage • If excitation voltage is lowered: • Voltage induced in A.C.
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